fix testnet prefix

[alekcangp]

Correct byte prefix for testnet BGL address: tbgl. This is applied to create BGL testnet wallet.

[MyTH-zyxeon]

Maintainer-facing review assist for the Bitgesell #81 improvement-program queue:

I rechecked this PR at head 86bf1d8356957b87afc6ec91406b5cb81fd0bcdf. It is a one-line src/constants.js change, currently OPEN/MERGEABLE/CLEAN, with no prior PR comments or reviews.

Suggested acceptance checks before merge:

• confirm the proposed TESTNET_SEGWIT_ADDRESS_BYTE_PREFIX byte sequence is the canonical testnet BGL/tbgl prefix for this library's address encoder, not only a visually plausible tbgl string target;
• add or point to a focused test vector that encodes and decodes one known testnet segwit address, so this does not silently change generated wallet addresses without regression coverage;
• verify mainnet segwit, mainnet script, testnet script, and private-key prefixes remain unchanged by the one-line patch;
• check compatibility with any published jsbgl package consumers before merging, since this changes wallet/address behavior rather than docs metadata;
• classify this under #81 as a correctness fix with wallet-address impact, not a docs-only cleanup, and avoid any live wallet/key handling during validation.

No live-chain action is needed to review this PR; the merge gate is deterministic address-prefix test coverage and maintainer confirmation of the canonical testnet prefix bytes.